chore: remove obsolete planning docs and test scripts

Removes historical planning documents and feature-specific test scripts
that are no longer needed now that features are implemented.

Removed:
- docs/plans/ directory (4 historical planning documents)
  - 2025-12-03-stack-removal-detection-design.md
  - 2025-12-04-stack-removal-cleanup.md
  - 2025-12-05-script-extraction-proposal.md
  - 2025-12-06-enhanced-stack-removal-detection-design.md
- scripts/testing/test-removal-detection.sh (feature-specific test)

Kept:
- scripts/testing/test-workflow.sh (general workflow testing)
- scripts/testing/validate-compose.sh (general compose validation)
- scripts/testing/README.md (testing documentation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: owine